In iron and steel, phosphorus (P), sulfur (S) and nitrogen (N) are deleterious ingredients, and it is well known that in stainless steels, if the contents of these ingredients are reduced, materials having excellent properties can be obtained. But it has been considered that dephosphorization of stainless steels, more broadly chromium-containing iron alloys, is extremely difficult; In particular oxidation dephosphorization thereof has been thought to be almost impossible. Therefore, when production of low phosphorus stainless steels is intended, specially selected low phosphorus materials are used instead of carrying out dephosphorization refining. Such specially selected materials are, of course, expensive.
As for the dephosphorization of plain pig iron, rather recently use of smelting slags containing oxides and/or carbonates of alkali metals has been proposed for the purpose of dephosphorization and desulfurization, for instance, in Japanese Laying-Open Patent Publication No. 28511/78 and "Tetsu-to-Hagane," Vol. 63,S157, 1977.
Further we developed a process for dephosphorizing chromium-containing pig iron (hereinafter referred to as "Cr pig iron"), using a slag comprising 30-80% by weight of at least one of fluorine and chloride of alkaline earth metals, 0.4-30% by weight of at least one of oxide and carbonate of lithium, 5-50% by weight of at least one of iron oxides and nickel oxide, and less than 40% by weight of at least one of oxide and carbonate of alkaline earth metals (Japanese Laying-Open Patent Publication No. 5910/81).
Although the process exhibits high refining performance, it cannot, however, be said to be economical, because a slag containing a lithium compound, which is expensive, volatile and thus low in utilization efficiency, is used. On the other hand, in the dephosphorization reaction occurring in the refining of the plain carbon steel, it is believed that the dephosphorization product is calcium phosphate containing no fluorine, and in fact dephosphorization occurs even when the CaF.sub.2 concentration is low. In the case of Cr pig iron, it is well known that dephosphorization hardly occurs with a CaO-iron oxide slag containing a very low concentration of CaF.sub.2.
We studied dephosphorization of Cr pig iron in order to find an economical dephosphorization process or a dephosphorization slag and to elucidate the dephosphorization mechanism using slags of various compositions. We tried to identify the dephosphorization product with the aid of X-ray diffractometry, and we have reached the conclusion that in the dephosphorization product there exists Ca.sub.5 F(PO.sub.4).sub.3 in addition to or instead of Li.sub.3 PO.sub.4, which we previously believed to be the main dephosphorization product.